Manufacturing process for an electric discharge lamp



Dec. 15, 1970 A. C. BOUCHARD ET AL MANUFACTURING PROCESS FOR AN ELECTRIC DISCHARGE LAMP vFiled Jan. 2, 1968 DISPERSE RESIN IN SOLVENT COAT INSIDE SURFACE OF LAM P ENVELOPE CURE RESIN FILM DRYAND OXIDIZE FILM FINISH WITH LAMP No FIG.3

ANDRE C.BOUCIHARD PETER w. GAUDET MARTHA J. .TH MA INVENQ'OFZS ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE In the manufacture of an arc discharge lamp having a silicone film on the inner surface of the lamp envelope, the envelope is heated in an atmosphere of nitrogen dioxide to oxidize and eliminate the organic constituents of the film.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electric discharge lamps which have an alkali-containing soft glass envelope, electrodes and mercury in the lamp fill and which can also have a phosphor coating on the envelope.

Description of the prior art Electric discharge devices, such as fluorescent or ultraviolet lamps, commonly undergo an undesirable diminution in their useful light output during life. When this decrease is expressed as the percentage ratio of the amount of light emitted after a certain period of operation to the amount emitted by the lamp when new, it is called lumen maintenance. Typical values for the maintenance of fluorescent lamps after several hundreds or thousands of operating hours are 70% to 90% i It has been determined that one of the causes of maintenance reduction is the formation of a dark mercuryalkali amalgam deposit on the inside of the glass envelope, the light transmission of which is thereby reduced. During operation, the mercury in the fill reacts with the alkali, especially sodium from the glass, to form the dark amalgam deposit.

' US. Pat. 3,205,394, issued Sept. 7, 1965 to Ray et al., discloses an aperture fluorescent lamp which had a particulate layer of silica on the inner enveolpe surface in order to inhibit the previously-mentioned mercury-alkali amalgam formation and improve the lamp maintenance. Since this silica layer was discontinuous, having been deposited; from asuspension of finely powdered silica in lacquer, alkali ions could still diffuse through the layer and react with the mercury vapor to form the dark amalgam; although the maintenance was far better than that of an uncoated lamp- An alternative method of inhibiting the migration of alkali ionswas disclosed in US. Pat. 3,094,641, issued June 18, 1963 to Gungle et al. The patent described a fluorescent lamp which had a soda-lime glass envelope, the composition of which included a small quantity of antimony trioxide. The antimony inhibited the diffusion of alkali ions to the surface of the glass and thereby reduced the amountof mercury-alkali amalgam formation and improved the lamp maintenance.

, In a copending application of Bouchard et al. entitled -Elect'ric Discharge Lamp Having a Continuous Three- Dimensional Coating on the Inner Surface of the Envelope, S.N. 670,268 filed Sept. 25, 1967 and assigned to the instant assignee, a three-dimensional film of crosslinked silicon and oxygen atoms on the inner surface of a lamp envelope is disclosed. The purpose of the film is to 3,547,680 Patented Dec. 15, 1970 ICC attain improvements in lamp maintenance by inhibiting the formation of a light-reducing mercury-alkali deposit.

The method of forming the film, which is therein disclosed, involves the oxidative degradation of a polyorganosiloxane film in order to eliminate the organic constituents. Oxidizing the film in an atmosphere of air required comparatively high temperatures and long periods of time to obtain substantially complete elimination of the organic constituents. One example cited required a temperature of 500 C. for 72 hours.

The instant invention relates to an improvement in the process of oxidatively degrading the polyorganosiloxane film.

SUMMARY OF THE INVENTION In our invention, an electric discharge lamp has a continuous film of three-dimensional, cross-linked Si-OSi linkages. The use of nitrogen dioxide in face of a glass envelope. Expressed another way, the film is a three-dimensional, cross-linked network of silicon and oxygen atoms in the ratio of approximately two oxygen atoms per silicon atom. The continuity of the film renders it more impervious to the diffusion of alkali ions than does the particulate coating of the prior art lamps and thereby reduces the amount of amalgam formation and improves the maintenance of the lamp. The preferred film thickness is less than about one micron.

In order to form the continuous film, a cross-linking polyorganosiloxane resin is dissolved in a solvent and the solution is applied to the inside of the glass envelope to form a thin coating or film. The film is then dried and cured at an elevated temperature to polymerize the resin. The polymerized resin is then carefully oxidized in an atmosphere containing at least 1% nitrogen dioxide at a temperature of about 250 to 550 C., but preferably at about 400 C., for several minutes to eliminate the organic radicals without rupturing the three-dimensional -Si-O-Silinkages. The use of nitrogen dioxide in the oxidizing atmosphere unexpectedly reduces, to a considerable extent, the time and/ or temperature required to eliminate the organic radicals. D uring oxidation, additional cross-linking occurs through the substitution of linking O-"atoms for the organic radicals.

Three-dimensional polyorganosiloxanes having the basic structure I where R is an organic group having 6 or less carbon atoms, preferably methyl, are included in the group of resins that generally can form satisfactory films for the purpose of this invention. Additionally in the preferred resins, the ratio of R groups toSi atoms is between about 0.5 and 1.8 and the ratio of O atoms to Si atoms is about 1.5.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view, partly in section, of a fluorescent lamp manufactured according to this invention. FIG. 2 is an enlarged cross-sectional view taken on line 2-2 of FIG. 1.

FIG. 3 is a flow sheet of the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT the operation ofthe lamp.. 0n the inside surface of CIIVC:

lope 2, there is a continuous film 3 of cross-linked SiOSi groups and a coating 4 of phosphor, as shown in enlarged section in FIG. 2.

At each end of envelope 1 there is an electrode comprising an oxide-coated tungsten coil 7, two auxiliary anodes 8, 9 and support and lead-in wires 10, Has shown, for example, in US. Pat. 2, 961,366, issued Nov. 22, 1960 to Waymouth et al. The usual insulating plastic base 12, with boss 13 carrying contacts 14, 15 can be as shown in US. Pat. 2,896,187, issued July 12, 1959 to Thomas et al.

Glass envelope 2 is usually made of a soda-lime glass having a sodium oxide (Na O) content greater than about A typical composition, expressed as the oxide, is

During normal lamp processing, glass envelope 2 is baked at a temperature of about 550 C. to remove the binder from the phosphor. In addition, during evacuation, the envelope is reheated to approximately 300 C. to facilitate outgassing of the internal parts of the lamp. Each of these bakeouts tends to diffuse the alkali from the glass of envelope 2 to the surface. During normal lamp operation, mercury ions can combine with the surface alkali to form the dark amalgam deposit on the glass which reduces the light transmission. Phosphor layer 4 is not a continuous film and, alone, is an inadequate barrier to the metallic ions to prevent the amalgam formation. The prior art particulate silica coating, mentioned in US. Pat. 3,205,394, supra, reduced the rate of formation and thereby improved the maintenance of the lamp. However, the unique continuity of film 3 is an even greater barrier to the passage of metallic ions and improves the lamp maintenance to a greater extent.

In the first step of the process, as illustrated in FIG. 3, a suitable silicone resin is dissolved in a solvent such as butanol or acetone. In order that the resultant film be three-dimensional and continuous, the resin used must be a cross-linking polyorganosiloxane having the basic structure where R is an organic functional group, such as phenyl or ethyl but preferably methyl. A white powdered polymethyethoxysiloxane resin in which the silicon and oxygen constituted about 89% of the total resin weight and in which the ratio of R groups to silicon atoms was about 1.5 proved satisfactory.

In a specific example of the process, a solution of 1% by weight of the white powdered resin in butanol was ap plied to the inner surface of envelope 2 by flushing, a technique common to the industry. After drying, to eliminate the solvent, the adhering resin film was cured at 200 C. for two hours to polymerize the resin to a higher molecular weight. Envelope 2 with adherent film 3 thereon was then oxidized in nitrogen dioxide at 500 C. for about five minutes to eliminate the organic groups from film 3. The flow rate of nitrogen dioxide through envelope 2 during oxidation was 800 milliliters per minute. The lamp was then completed by attaching the ends, exhausting and sealing, as is conventional in the art.

The purpose of the oxidation at an elevated temperature is to eliminate the organic constituents without rupturing the three-dimensional -SiOSinetwork. Although the exact mechanism of the reaction is not known, it is postulated that the carbon and hydrogen of the organic radicals are oxidized to CO or CO and H 0, respectively, and an -O- atom is substituted for the organic radical, which results in additional cross-linking. We have found that accurate control of the time and temperature of oxidation is required in order to obtain the desired continuous film and still eliminate substantially all the organic constituents. Baking in nitrogen dioxide at 500 C. for five minutes was suflicient in the example described. Below about 200 to 250 C., the reaction is too slow to be practical. The upper temperature limit is usually governed by the softening point of the substrate, which is about 550 to 600 C. for soda-lime glass envelope 2.

It is not essential that the oxidizing atmosphere consist entirely of nitrogen dioxide, since mixtures of nitrogen dioxide and other gases, such as oxygen, nitrogen or air, can also reduce the time and/or temperature of the baking process necessary for complete elimination of the organic constituents of the film. For example, when nitrogen dioxide, at a flow rate of 200 cubic centimeters per minute, and air, at a fiow rate of 14,000 cubic centimeters per minute, were passed through envelope 1 which was at a baking temperature of 500 0., about 20 minutes were sufiicient to oxidize and eliminate the organic constituents. At flow rates of '800 cubic centimeters of nitrogen dioxide per minute and 7,000 cubic centimeters of oxygen per minute, about 10 minutes Were sufiicient. Thus, it is seen that nitrogen dioxideconcentrations as low as 1% of the baking atmosphere can materially reduce the necessary baking time.

The reasons for this improvement are not completely understood since the oxidation reduction potentials for the reduction of nitrogen dioxide do not indicate that nitrogen dioxide is a stronger oxidizing agent than oxygen. However, it is believed that the nitrogen dioxide may dissociate into nitrous oxide and nascent oxygen, at the temperatures involved in the process and that the nascent oxygen, a powerful oxidizing agent, is responsible for the marked reduction in the time required for comlete oxidation.

The composition of the organic groups in the polyorganosiloxane resin also affects their rate of removal during baking. When R in the above resin formula is predominantly phenyl, longer times and higher temperatures are necessary to eliminate substantially all the organic constituents. Conversely, when R is predominantly ethyl, substantially complete elimination is more easily attained than when R is predominantly methyl. However, the film resulting from polyethylsiloxane resin is more susceptible to crazing or cracking than that from polymethylsiloxane. Therefore, the latter is preferred. The ratio of organic groups to silicon atoms in the resins capable of forming continuous films can vary from about 0.05 to 1.8.

Substantially complete elimination of the organic groups is determined by infrared spectrum analysis or thermal gravimetric analysis. In former, the infrared transmittance curve of the desired film is similar to that of vitreous silica or quartz. In the latter, oxidation degradation at even higher temperatures yields no perceptible weight loss.

It has been determined that linear polyorganosiloxanes, having the structure will not yield the desired continuous film when they are cured and oxidized according to this process, since there will be substantially no three-dimensional cross-linking. The residue is merely a coating of finely powdered an-v hydrous silica. Organic silicates, also, yield a similar discontinuous film.

The combination of the mercury with the alkali which forms the light reducing deposit, mentioned previously, appears to occur within the region extending from the surface of the glass throughout the thickness of the phosphor coating. The normal baking cycles of lamp processing cause the alkali ions within the glass to diifuse toward the inner surface of the envelope, thereby making the inner surface alkali-rich. The discontinuities in the phosphor coating, in addition to those in the particulate silica coating of the prior art, permit a rapid diffusion of both mercury and alkali ions, since the difiusion .is essentially in the voids between the particles and hence has a rate approaching that of gaseous diffusion. Thus the combination of the alkali with the mercury ions can occur anywhere within the above-mentioned region with the result that a black-brown mercury-alkali amalgam can be deposited anywhere within this region, that is, on the surface of the glass and/ or on the particles of phosphor. In any case, the dark deposit reduces the light transmission. In addition, the dark deposit on the phosphor particles can decrease the amount of 2537 A. radiation reaching the phosphor and thereby reduce the amount of visible light emitted by the phosphor.

The continuous three-dimensional film is a greater barrier to the diffusion of alkali ions than is the particulate coating of the prior art. The diffusion of the ions through the film is essentially solid state diffusion and proceeds at a much slower rate than the essentially gaseous diffusion through the particulate coating. Thus, when the greatest amount of alkali diffusion occurs, which is at the elevated temperatures of the usual lamp processes, previously mentioned, fewer alkali ions will have diffused to the inner surface of the glass envelope of our invention.

One test to determine the effectiveness of the barrier layer is to measure the amount of water soluble sodium which has diffused to the inner surface of the glass after the usual baking steps. Normally the unbaked envelope when rinsed with water, yields no measurable amount of water soluble sodium. However the same rinsing technique on a baked envelope results in measurable amounts of sodium. Table II shows the results on a glass envelope coated with the continuous film and on an uncoated glass envelope, after both were baked at 550 C. for one hour.

TABLE II Micrograms Na Uncoated 95 Coated 21 TABLE III Film thickness, Soluble Na,

microns micrograms Control envelope (no coating). 154 3% by weight of resin in coating solution. 0. 39 46 1% by weight of resin in coating solution- 0. 27 39 0.1% by weight of resin in coating solution 0. 11 60 The above shows that the desired film thickness, for the particular envelope tested, appears to be about 0.10 to 0.40 micron with the optimum at about 0.30 micron. A thinner film has a decreased eifect in reducing the rate of diffusion of alkali ions. In the case of thicknesses greater than about 1 micron, crazing of the film results TABLE IV Percent maintenance 0 hour hours 500 hours Uncoated 100 89. 2 77. 9 Coated 100 96. 0 91. 7

Although this invention has been described particularly in relation to fluorescent lamps, it can also be used in other discharge devices having alkali in the glass envelope and mercury in the fill. An example is an ultraviolet lamp which does not have a phosphor. A mercuryalkali amalgam can deposit on the inside surface of the envelope and reduce the transmission of ultraviolet radiation during the life of the lamp. The continuous film deposited and processed according to our invention would also increase the maintenance of the ultraviolet lamp.

It is apparent that modifications and changes can be made within the spirit and scope of the instant invention.

We claim:

1. In the manufacture of a discharge device having a fill including mercury and having an alkali-containing glass envelope, the steps which comprise:

coating the inner surface of said glass envelope with a cross-linking polyorganosiloxane resin solution to form a resin film thereon;

drying and curing said film; and

heating the cured film in contact with an oxidizing atmosphere including at least 1% nitrogen dioxide, whereby the organic groups of said resin film are oxidized and substantially eliminated from said film.

2. The process of claim 1 wherein said envelope is a tube and said atmosphere of nitrogen dioxide is introduced into one end of said envelope, flows over said resin film and exits from the other end.

3. The process of claim 1 wherein said heating temperature is between about 250 and 550 C.

4. The process of claim 1 wherein the ratio of organic groups to silicon atoms in said polyorganosiloxane resin is between about 0.5 and 1.8, and said organic groups are predominantly methyl.

References Cited UNITED STATES PATENTS 2,295,626 9/ 1942 Beese 313-221X 2,778,793 1/ 1957 Thomas et al. 117-62X 2,838,707 6/1958 Schwing et al. 313-l09 3,018,187 1/1962 Boyce et al 313-109X 3,094,641 6/ 1963 Gungle et al. 313109 3,205,394 9/1965 Ray 313109 3,377,494 4/1968 Repscher 313-109 3,414,433 12/1968 Van Bramer 117106X WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner US. Cl. X.R. 

